The Video Codec

The video codec is a device which takes composite video and audio, digitizes it, compresses it and transmits the frames to another codec.

The VistaCom video codec uses the ITU-T (formerly CCITT) H.261 compression, so it is able to communicate with codecs from other manufacturers also.

Although the codec is able to utilize line speeds of up to 2Mbps, it can also operate on low speeds, like the 64kbps dial-up satellite link. In that case we would typically use 40kbps for video, 16 for audio and the rest would be used by CSCW applications. The picture resolution of the codec is CIF (=Common Interchange Format), which is 352 x 288 for luminance and half of that for chrominance both vertically and horizontally. This means that there is only 1/4 of color information of the complete black-and-white picture. This is adequate in many cases. The standard also recognizes a QCIF format which is quarter of the resolution of CIF.

You can see the quality of the picture of the video codec. Take a look at my office.

About H.261

H.261 is a compression algorithm like MPEG. It even resembles MPEG a bit. The greatest difference is that H.261 has been optimized for lower line speeds than MPEG. H.261 is usable even on a 64 kilobit channel, where MPEG is defined to use 0.9 - 1.5 Megabits.

Anyway, just like MPEG, also H.261 is very expensive in terms of CPU time to encode and decode. That's why it is often done with special-purpose hardware, like the VistaCom video codec.

But since not everyone has or can afford that hardware, there are programs that will do the job of video codecs, although much slower.

I have prepared a short demo of moving video. If you haven't used Mosaic for H.261 viewing before, there are a few things you must do.

H.261 viewing instructions

These instructions are based on the assumption that your WWW browser is NCSA Mosaic for X Window System. If that is not the case, please contact me and maybe we can work something out.

The users of the Computer Science laboratory at Helsinki University of Technology can jump directly to item 3 on the following instructions.

Get H.261 decoding software. If you don't know where to find such a beast, look at the following lines.
- Turn on Load to local disk from the Mosaic Options menu
- Click on either one of the two lines below
  1. The source code for a simple H.261 decoder (~900 KB).
  2. Already compiled executable for Sun Sparcstations (~45 KB).
- Turn off Load to local disk from the Mosaic Options menu.
Compile and install the H.261 decoder
Add the following line to your ~/.mailcap file:
```
video/x-h261; h261_decode -f %s
```
Choose Reload config files from the Options menu

If you have completed the above tasks, you may now click this to continue.

If for some odd reason you prefer MPEG, I have converted the video clip to MPEG (~350 KB) for your convenience.

Sami Tikka